Hepatic cell types are being separated for the purpose of determining their capacity to produce Ep. These procedures include in situ perfusion of normal and regenerating rat livers with a solution of collagenase and hyaluronidase and redigestion of the cell suspension with a solution of pronase and DNAase. With these methods, cultures containing 97-99% viable phagocytic Kupffer (K) cells are obtained. Preliminary data indicate a capacity of these relatively uniform suspensions of K cells to produce Ep in vitro. Factors such as thyroid hormones, adenohypophyseal growth hormone (GH), cyclic nucleotides and prostaglandins, which have been shown to influence renal Ep production and to affect reticulo-endothelial function, will be tested for their ability to cause these cultured cells to produce Ep. Perfusion of regenerating livers at 48-96 hr after subtotal hepatectomy has offered a simple practical method for obtaining the hepatopoietic factor(s) (Hp) which triggers Ep production in nephrectomized rats subjected to hypoxia (Naughton et al., 1977, 1978). The precise site(s) of extrarenal production of Ep are being located by the ferritin labeling antibody technique, utilizing purified rat Ep as the antigen and a purified antibody to rat Ep developed in guinea pigs.